本文仅探讨s3c6410从nand flash启动u-boot时的代码重定位过程
参考:
1)《USER'S MANUAL-S3C6410X》第二章 MEMORY MAP 第八章 NAND FLASH CONTROLLER
2)u-boot源码:
u-boot-x.x.x/board/samsumg/smdk6410/lowlevel_init.S
u-boot-x.x.x/cpu/s3c64xx/start.S
u-boot-x.x.x/cpu/s3c64xx/nand_cp.c
代码重定位过程简述
由于在nand flash中无法运行代码,所以当开发板从nand flash启动时,我们需要将存储在外设nand flash中的u-boot代码搬运到sdram中运行,如何完成这个搬运工作呢?这需要借助一个跳板,即“stepping stone”,它是s3c6410的一块内置sram,开发板上电时,nand flash控制器自动将nand flash的前8K的内容拷贝到sram中并执行,这一小段启动代码除了初始化硬件外,最重要的一个工作就是将nand flash中的所有u-boot代码拷贝(即重定位)到sdram的指定地址上去,然后跳转到sdram中执行。
重定位代码解析:
1)nand接口初始化
u-boot启动时,首先执行相应硬件平台的start.S,start.S中调用lowlevel_init对时钟,uart,nand,mmu等底层硬件作初始化。
start.S:
...
bl lowlevel_init /* go setup pll,mux,memory */
...
lowlevel_init.S:
...
/*
* Nand Interface Init for SMDK6400 */
nand_asm_init:
ldr r0, =ELFIN_NAND_BASE
ldr r1, [r0, #NFCONF_OFFSET]
orr r1, r1, #0x70
orr r1, r1, #0x7700
str r1, [r0, #NFCONF_OFFSET]
ldr r1, [r0, #NFCONT_OFFSET]
orr r1, r1, #0x03
str r1, [r0, #NFCONT_OFFSET]
mov pc, lr
...
2)代码重定位
从nand flash启动时,重定位代码如下:
start.S:
/* when we already run in ram, we don't need to relocate U-Boot.
* and actually, memory controller must be configured before U-Boot
* is running in ram.
*/
ldr r0, =0xff000fff
bic r1, pc, r0 /* r0 <- current base addr of code */
ldr r2, _TEXT_BASE /* r1 <- original base addr in ram */
bic r2, r2, r0 /* r0 <- current base addr of code */
cmp r1, r2 /* compare r0, r1 */
beq after_copy /* r0 == r1 then skip flash copy */
#ifdef CONFIG_BOOT_NAND
mov r0, #0x1000
bl copy_from_nand
#endif
r1存放当前代码运行的起始地址,r2存放u-boot即将在sdram中运行的地址,如果两个地址相等,说明此时u-boot已经在sdram中运行了,无需再执行从nand拷贝数据到sdram的动作;否则,此时u-boot还在它的临时住所sram中执行,此地不可久留,需要执行copy_from_nand将u-boot代码完完整整地拷贝到sdram中去,然后跳转到sdram中去执行剩下的代码。
/*
* copy U-Boot to SDRAM and jump to ram (from NAND or OneNAND)
* r0: size to be compared
* Load 1'st 2blocks to RAM because U-boot's size is larger than 1block(128k) size
*/
.globl copy_from_nand
copy_from_nand:
mov r10, lr /* save return address */
mov r9, r0
/* get ready to call C functions */
ldr sp, _TEXT_PHY_BASE /* setup temp stack pointer */
sub sp, sp, #12
mov fp, #0 /* no previous frame, so fp=0 */
mov r9, #0x1000
bl copy_uboot_to_ram
3: tst r0, #0x0
bne copy_failed
ldr r0, =0x0c000000
ldr r1, _TEXT_PHY_BASE
1: ldr r3, [r0], #4
ldr r4, [r1], #4
teq r3, r4
bne compare_failed /* not matched */
subs r9, r9, #4
bne 1b
4: mov lr, r10 /* all is OK */
mov pc, lr
copy_failed:
nop /* copy from nand failed */
b copy_failed
compare_failed:
nop /* compare failed */
b compare_failed
真正执行拷贝动作的是copy_uboot_to_ram函数,它定义在u-boot-x.x.x/cpu/s3c64xx/nand_cp.c中,
int copy_uboot_to_ram (void)
{
int large_block = 0;
int i;
vu_char id;
NAND_ENABLE_CE();
NFCMD_REG = NAND_CMD_READID;
NFADDR_REG = 0x00;
/* wait for a while */
for (i=0; i<200; i++);
id = NFDATA8_REG;
id = NFDATA8_REG;
if (id > 0x80)
large_block = 1;
/* read NAND Block.
* 128KB ->240KB because of U-Boot size increase. by scsuh
* So, read 0x3c000 bytes not 0x20000(128KB).
*/
return nandll_read_blocks(CFG_PHY_UBOOT_BASE, 0x3c000, large_block);
}
nand flash支持两种页大小,512B和2KB,large_block = 0时,页大小为512字节,large_block = 1时,页大小为2K字节。nandll_read_blocks拷贝nand flash从第0页开始的0x3c00(240K)大小的数据到sdram的CFG_PHY_UBOOT_BASE地址处。
/*
* Read data from NAND.
*/
static int nandll_read_blocks (ulong dst_addr, ulong size, int large_block)
{
uchar *buf = (uchar *)dst_addr;
int i;
uint page_shift = 9;
if (large_block)
page_shift = 11;
/* Read pages */
for (i = 0; i < (0x3c000>>page_shift); i++, buf+=(1<
}
return 0;
}
首先根据large_block判断nand flash一个页的大小,从而计算需要拷贝的页的数量,即需要拷贝(0x3c000>>page_shift)个页,nandll_read_page每次只拷贝一个页的数据。
/*
* address format
* 17 16 9 8 0
* --------------------------------------------
* | block(12bit) | page(5bit) | offset(9bit) |
* --------------------------------------------
*/
static int nandll_read_page (uchar *buf, ulong addr, int large_block)
{
int i;
int page_size = 512;
if (large_block)
page_size = 2048;
NAND_ENABLE_CE();
NFCMD_REG = NAND_CMD_READ0;
/* Write Address */
NFADDR_REG = 0;
if (large_block)
NFADDR_REG = 0;
NFADDR_REG = (addr) & 0xff;
NFADDR_REG = (addr >> 8) & 0xff;
NFADDR_REG = (addr >> 16) & 0xff;
if (large_block)
NFCMD_REG = NAND_CMD_READSTART;
NF_TRANSRnB();
/* for compatibility(2460). u32 cannot be used. by scsuh */
for(i=0; i < page_size; i++) {
*buf++ = NFDATA8_REG;
}
NAND_DISABLE_CE();
return 0;
}
从nand flash中读取数据的流程为片选(NAND_ENABLE_CE)->发读命令(NFCMD_REG)->发地址(NFADDR_REG)->发读命令(NFCMD_REG)->等待数据可读(NF_TRANSRnB)->读数据(NFDATA8_REG)。由于每次从NFDATA8_REG中只可读取1个字节的数据,所以拷贝一页需要读取512或2048次。
当执行完copy_uboot_to_ram返回到start.S时,nand flash中的代码重定位便完成了,此后程序跳转到sdram中执行,stepping stone的职责就此结束。